In connection with Eq. (REF )
we should note that multiple
nontrivial technical problems may
emerge when the dimension of our
topological vector space
\({\cal V}\)  of kets is infinite.
This aspect of the theory
must carefully be discussed
in the framework of functional analysis [1]}.
As a complementary further reading
in this direction let us mention
the old Dieudonné's paper [2]},
or the recent Refs. [3]} and
[4]}.
This being said, it is useful to add, explicitly, that
in any case, the Hilbert-space metric
operator \(\Theta \)  must necessarily be chosen
invertible and positive definite and bounded,
with bounded inverse [5]}, [1]}.
